NASA postponed the launch of space shuttle Discovery on Saturday due to thunderstorms and the possibility of lightning in the area of the Florida launch pad.
The US space agency will try again on Sunday to launch a mission whose failure could ground the US shuttle fleet permanently and leave the International Space Station unfinished.
"It's not a good day to launch," said NASA flight director Steve Stich.
Discovery's mission is just the second since the destruction of the shuttle Columbia and the deaths of seven crewmembers in 2003, an accident triggered by insulating foam that fell off the spacecraft's fuel tank. NASA has since spent $1.3 billion on repairs and upgrades to the shuttle.
Discovery's mission is meant to test repairs to the fuel tank, carry much-needed equipment and supplies to the space station and make repairs to the $100 billion orbiting outpost.